I see the same problem. My machine has an Intel Core i7-7567U @ 3.5Gz x 4, running Pop-OS (really Ubuntu) 19.04, with Zoiper 5 installed about two weeks ago. I'm seeing over 5% CPU from Zoiper during idle. There are two zoiper processes, one using 4.7%, and the other 0.7%. This is idle. No call. This is too much CPU!
There is something in Zoiper software that is looping much too quickly. A status check, or GUI refresh, or something that should NOT be running without reasonable idle time of at least 500 milliseconds or several seconds per loop.
Wasted electricity, wasted heat, and wasted time listening to my CPU fan cycle too often. Why should one
idle app rob my computer of 5% power at all times? Here's a copy of my 'top' output. (Easier to read if you view with a monospaced font.) Zoiper is at the top almost all the time.
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
21991 cmjanic+ 20 0 3013440 200204 89928 S 4.7 1.2 10:10.38 zoiper
1669 cmjanic+ -2 0 3373896 402884 92652 S 3.3 2.5 19:47.57 gnome-sh+
1485 cmjanic+ 20 0 657140 305856 247960 S 1.7 1.9 26:04.09 Xorg
15112 cmjanic+ 20 0 3079288 397508 218600 S 1.7 2.4 4:13.32 Web Cont+
2308 cmjanic+ 20 0 575348 46360 31484 S 1.3 0.3 0:29.54 gnome-te+
21945 cmjanic+ 20 0 2473796 113720 81540 S 0.7 0.7 1:16.13 zoiper
1058 gdm 20 0 901280 16004 12204 S 0.3 0.1 0:01.91 pulseaud+
1888 cmjanic+ 39 19 1183228 98588 13308 S 0.3 0.6 1:27.78 tracker-+
5029 cmjanic+ 20 0 2399080 368284 126808 S 0.3 2.3 1:43.65 thunderb+
7450 cmjanic+ 20 0 4074984 816108 311940 S 0.3 5.0 69:10.88 firefox
19014 cmjanic+ 20 0 8251128 369640 44692 S 0.3 2.3 1:35.80 java
26528 cmjanic+ 20 0 21656 3804 3120 R 0.3 0.0 0:00.22 top
1 root 20 0 166328 9156 6304 S 0.0 0.1 0:28.75 systemd
2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kthreadd
3 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 rcu_gp
4 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 rcu_par_+