Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 122798)
+++ channels/chan_sip.c	(working copy)
@@ -16841,6 +16841,12 @@
 			ast_copy_string(peer->mohsuggest, v->value, sizeof(peer->mohsuggest));
 		} else if (!strcasecmp(v->name, "mailbox")) {
 			ast_copy_string(peer->mailbox, v->value, sizeof(peer->mailbox));
+		} else if (!strcasecmp(v->name, "hasmailbox")) {
+			/* People expect that if 'hasmailbox' is set, that the mailbox will
+			 * be also set, even if not explicitly specified. */
+			if (ast_true(v->value) && ast_strlen_zero(peer->mailbox)) {
+				ast_copy_string(peer->mailbox, name, sizeof(peer->mailbox));
+			}
 		} else if (!strcasecmp(v->name, "subscribemwi")) {
 			ast_set2_flag(&peer->flags[1], ast_true(v->value), SIP_PAGE2_SUBSCRIBEMWIONLY);
 		} else if (!strcasecmp(v->name, "vmexten")) {