Many questions are being asked to clarify how many modules can be fitted on Yeastar ( old Yealink ) PBXes. See the calculations below.
S20 can directly have 2 Modules ( each module can have either 2 FXO or 2FXS , or 1FXO and 1FXS, or 1GSM ) – see the module pictures below.
S20 can directly have 2 Modules – so 4 FXO
S20 can directly have 4 Modules – so 8 FXO
S100 can carry 2 X EX08 module – ( mean 8 Modules , that is 16 FXO lines )
S300 can carry 3 X EX08 module – ( mean 12 Modules , that is 24 FXO lines )