I have both PlusTalk and DrayTel SIP accounts setup.
The PlusTalk phone can ring the Draytel Phones via SIP and by using POTS without any issue.
The DrayTel phones however cannot ring into the PlusTalk phone without getting the engaged tone.
The DrayTel phones can however ring into the PlusTalk phone via POTS without the engaged problem as can my PSTN phones.
I'm using the PlusTalk phone via ATA port 2 on a DrayTek Vigor 2600VG Router with a DrayTel phone on ATA port 1. I also have a second DrayTel phone attached to a VigorTalk ATA adapter.
I have also asked others to dial into the PlusTalk phone via SIP and they receive the engaged tone also. (All DrayTel Phones since all collegues are using DrayTel) It may also be an issue with other PlusTalk SIP phones, but I don't have a second PlusTalk account avilable.