I need to install one package. Where do I get dependencies for Ubuntu 16.04.3 x64?

Following dependencies are not found.

root@ubuntu-s-1vcpu-1gb-ams2-01:~# apt-get install libopal-dev
Reading package lists... Done
Building dependency tree 
Reading state information... Done
E: Unable to locate package libopal-dev
root@ubuntu-s-1vcpu-1gb-ams2-01:~# apt-get install libpt-dev libopal-dev
Reading package lists... Done
Building dependency tree 
Reading state information... Done
E: Unable to locate package libpt-dev
E: Unable to locate package libopal-dev
root@ubuntu-s-1vcpu-1gb-ams2-01:~# 



root@ubuntu-s-1vcpu-1gb-ams2-01:~/sipcmd# make
g++ -c -Wall src/main.cpp -o src/main.o -I/usr/include/opal -I/usr/include/ptlib -Isrc/ -g -DDEBUG
In file included from /usr/include/opal/h323/h323.h:39:0,
 from src/includes.h:25,
 from src/main.h:24,
 from src/main.cpp:24:
/usr/include/opal/h323/h323ep.h:148:30: warning: converting to non-pointer type 'unsigned int' from NULL [-Wconversion-null]
 unsigned int options = NULL, ///< options to pass to conneciton
 ^
src/main.cpp: In member function 'bool Manager::SendDTMF(const PString&)':
src/main.cpp:446:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (; i < dtmf.GetSize() - 1; i++) {
 ^
src/main.cpp:459:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 ok = (i == dtmf.GetSize() - 1 ? true : false);
 ^
g++ -c -Wall src/commands.cpp -o src/commands.o -I/usr/include/opal -I/usr/include/ptlib -Isrc/ -g -DDEBUG
In file included from /usr/include/opal/h323/h323.h:39:0,
 from src/includes.h:25,
 from src/main.h:24,
 from src/state.h:25,
 from src/commands.cpp:29:
/usr/include/opal/h323/h323ep.h:148:30: warning: converting to non-pointer type 'unsigned int' from NULL [-Wconversion-null]
 unsigned int options = NULL, ///< options to pass to conneciton
 ^
src/commands.cpp: In member function 'virtual bool Wait::ParseCommand(const char**, std::vector<Command*>&)':
src/commands.cpp:439:30: warning: format '%u' expects argument of type 'unsigned int*', but argument 3 has type 'size_t* aka long unsigned int*' [-Wformat=]
 sscanf(*cmds, "%u", &millis);
 ^
g++ -c -Wall src/channels.cpp -o src/channels.o -I/usr/include/opal -I/usr/include/ptlib -Isrc/ -g -DDEBUG
In file included from /usr/include/opal/h323/h323.h:39:0,
 from src/includes.h:25,
 from src/channels.h:25,
 from src/channels.cpp:27:
/usr/include/opal/h323/h323ep.h:148:30: warning: converting to non-pointer type 'unsigned int' from NULL [-Wconversion-null]
 unsigned int options = NULL, ///< options to pass to conneciton
 ^
g++ src/main.o src/commands.o src/channels.o -o sipcmd -lopal -lpt 

Generally speaking: Dependencies for packages can easily be found for supported packages by reviewing the appropriate page in Ubuntu Package Search.

Specifically speaking: The Ubuntu dependencies for the
tmakkonen/sipcmd project (last updated November 2016) you've linked to are listed as libpt-dev & libopal-dev both of which are available after enabling the Universe repository as indicated by @muru in a comment above. Once you have enabled the required repository for the packages you need, your apt-get install commands will work as advertised.

Following worked

make outputs are just WARNING (which was confusing).

$ cd /root
$ apt-get install libopal-dev
$ git clone https://github.com/tmakkonen/sipcmd.git
$ make

$ ./sipcmd -P sip -u 244xxx -c 1234 -w sip1.nomado.eu -x "c0012127773456;ws2000;d**;w200;h;j4"

Description:

- login to 244xxx with password 1234 at sip1.nomado.eu
- call 0012127773456
- wait for 2000 ms
- send DTMF : * * 
- wait for 200 ms
- hangup