Add a line Sipaddheader(X-DSTPHONE:${E}); before lines containing zabbix.
sed '/zabbix/a Sipaddheader(X-DSTPHONE:${E});' extensions.ael > extensions2.ael
Add the same but starting with 2 tabs seems to be not hard. But the problem is that not all sed versions support \t parameter.
So, neither of these worked for me:
sed '/zabbix/a \t\tSipaddheader(X-DSTPHONE:${E});' extensions.ael > extensions2.ael
sed '/zabbix/a \t\t Sipaddheader(X-DSTPHONE:${E});' extensions.ael > extensions2.ael
sed '/zabbix/a\t\t Sipaddheader(X-DSTPHONE:${E});' extensions.ael > extensions2.ael
The solution is as follows: insert a literal tab pressing Ctrl-V and then Tab (not shown in the listing).