You can use a AudioCodes MP-104/FXO/AC/xxx for 4 ports or a MP-108/FXO… for 8 ports. The most common protocol in EU is H.323, SIP or other are also available. The MPs also support T.38 fax (important for business customer).
If you’re looking for a VoIP Gateway for the Corporate market then you can’t go far wrong with the Quintum range of products. The Tenor A400 (4 port analogue) and A800 (8 port analogue) are FXO and FXS in the same box – this solves a lot of problems when deciding whether to connect to a PBX as either a trunk or extension (basically you can do either).
Another advantage is that with the Quintum you can route calls to the IP network or back to the PSTN – this is ideal for emergency calls (999, 911 etc.) which you would never want to route on IP. Also you can route back to the PBX if the IP network is conjested – this way you can “guarantee” voice quality to corporate clients.