I just came across a rather unusual situation where I could not place calls but I can receive them. This was with a Grandstream GRP2613 (FW 188.8.131.52) using VOIP.ms as the VoIP provider. After examining packet traffic I found unexpected information in the SIP header and traced it to some settings under Account–>SIP Settings–>Custom SIP Headers. Specifically, I disabled “Use X-Grandstream-PBX Header” and “Use P-Access-Network-Info Header” which default to be enabled. Please see image below.
Just wanted to share this in case anyone else runs into this problem.