Cannot make calls on MTN

Near the end of February, I experienced an issue where, on MTN,

Outbound (MO) calls ended shortly ("Call Ended") after dialing, except to numbers like 100, 123, and 124;

the output of *#33# was:

Call barring
All outgoing calls
Service has been disabled.

;

the issue persisted, even with *31#<number>;

credit was GHS3.89, or after adding GHS1 to see if it'll solve the issue, GHS4.89;

messages, inbound (MT) calls, data, and MoMo, were okay;

the issue affected 2G, 3G, and 4G;

outbound calls were normal with an AT SIM in the other SIM slot;

when the details of "call ended" were checked,

16:08:40.074 [NAS] CC SETUP
16:08:40.075 [WCDMA RRC] UplinkDirectTransfer
16:08:40.243 [WCDMA RRC] DownlinkDirectTransfer
16:08:40.243 [NAS] CC CALL PROCEEDING
16:08:40.793 [WCDMA RRC] DownlinkDirectTransfer
16:08:40.794 [NAS] CC DISCONNECT
16:08:40.794 [NAS] CC Cause [31]
16:08:40.794 [NAS] CC Cause [31]
16:08:40.794 [NAS] CC Cause [31]
16:08:40.794 [NAS] CC RELEASE
16:08:40.794 [WCDMA RRC] UplinkDirectTransfer
16:08:40.913 [WCDMA RRC] DownlinkDirectTransfer
16:08:40.914 [NAS] CC RELEASE COMPLETE
16:08:40.914 [NAS] CC Cause [31]
16:08:40.914 [NAS] CC Cause [31]
16:08:40.993 [WCDMA RRC] RRCConnectionRelease
16:08:40.994 [WCDMA RRC] RRCConnectionReleaseComplete
16:08:41.034 [WCDMA RRC] RRCConnectionReleaseComplete

.

The problem was solved by e-mailing customer support. Other details remain unknown.